A man has been attacked by a knife-wielding assailant on a train in south-east London. 

The victim has been taken to hospital with life-threatening injuries and a manhunt for the suspect is under way, police said.

Passengers watched as the attack unfolded and filmed the assault on their phones.

Witnesses said the attack had taken place in front of young children who had been left traumatised.

Graphic footage of the attack, which happened on a train near Beckenham Junction shortly before 4pm, was widely shared on social media.

In the video, a hooded man wearing a face mask is seen lunging at another person on the floor with a large knife.

One passenger, who appears to be calling the police, can be heard screaming “stop it now” before saying: “I’m on the train to Beckenham Junction and this man’s stabbing someone on the train.”

Commuters are heard calling the police and begging with the attacker to stop his assault.